The present invention relates to a digital data demodulator which is particularly, but not exclusively, suitable in MSK, TFM and GMSK signalling.
In frequency shift keying (FSK), a carrier signal f.sub.c is frequency modulated by a deviation frequency, for example +.DELTA.f for a "1" and -.DELTA.f for a "0". The digital data can be recovered by quadrature mixing f.sub.c .+-..DELTA.f with a local oscillator frequency of f.sub.c and low pass filtering, one channel (I) will produce the in-phase signal +.DELTA.f or -.DELTA.f and the other channel (Q) will produce the quadrature signal +.DELTA.f-.pi./2 or -.DELTA.f+.pi./2, respectively.
By comparing the filtered signals, one can determine the relative phase of the signal. If it leads by 90.degree. then it is, for example, a "1" and if it lags by 90.degree. then it is, for example, a "0".
This comparison can be done by hard limiting the respective signals and applying them to a D-type flip-flop--one as a strobe input and the other as the D input--see, for example, British Patent Specification No. 2,032,737A. Thus, a phase change indicates a transition from a "1" to "0" or vice versa. However, there is an acquisition delay. The delay can be reduced by having two D-type flip-flops, the D-inputs being connected to the respective I and Q channels and their clock inputs being connected to the channel not connected to their D-inputs. While this known technique may be satisfactory for FSK signalling it is not suitable for faster signalling rates as occurs in FFSK. Additionally, this known technique does not indicate how one can cope with the problem of the local oscillator frequency and the input carrier frequency being different which can lead to distortion of the signals. Accordingly, it is desirable to be able to adjust the local oscillator frequency so that it can track changes in the received carrier signal, in which case it is necessary to recover the carrier signal.